Volumes are created on a hard disk by providing parameters to disk creation utilities such as Fdisk on Windows NT or Disk Druid on Linux. The parameters includes information such as partition type, volume name and volume size. The disk utility creates the new volume by allocating the disk space as dictated by the user's given parameters. Therefore if the disk utility is instructed to create a 1 GB volume, 1 GB of disk space will be allocated and reserved for the volume's exclusive use. Once the disk space is allocated, the disk space belongs to that volume, and that volume only, whether data is written to it or not.
This method of creating volumes in which the required disk space must be pre-allocated is not very efficient in terms of cost. From an accounting standpoint, a resource costs less to own and maintain if it is used rather than if the same resource is standing by empty. Unused resources serve no purpose. In terms of storage devices, a large percentage of the disk space will remain unused until users can generate enough data to utilize the entire capacity. An analogy to this inefficiency is similar to creating and stocking large amounts of a product in anticipation of consumers wanting to buy the product. If the product is in great demand, then the necessity of creating and stocking is worth the effort because the product will be consumed very quickly. There is minimal inventory cost to maintain the storage and upkeep of the product. However, if due to the nature of the product, consumers slowly purchase the product over time, the inventory cost in stocking the product is very high. Storage fees will need to be paid to store the product in a warehouse, maintenance fees will need to be paid if the product can deteriorate if a proper environment is not maintained, and the product can lose its usefulness to the consumer if the product “sits on the shelf” for too long. A more efficient model is to create the product as needed and to maintain a very small inventory in the event of a sudden surge in the consumption of the product.
Disk space in a server environment also suffers from the above issues. The consumers are the users on a network who utilizes the disk storage on servers. A volume's disk space is the product that users consume. In general, users do not consume a volume's entire available disk space in a short time, but rather a first amount of disk space is used when the volume is initially created to store their applications and data files, and then the disk space is used in small amounts over an extended period of time as new files are created or as old files grow larger. Data is more likely to be “read from” than “written to” a disk. Therefore large amounts of disk space can remain unused for a long period of time.
An administrator tends to create a volume with much more capacity than is initially required because it is very difficult to predict the exact usage of a volume since disk space usage is very dynamic. New users may be added. Old users and their data may be deleted. Some users may need very little additional storage after their initial applications and data files are stored, while other users may require a large amount of storage in an instant. The administrator must take into account all these factors and will generally over allocate the disk storage.
Veritas' Volume Manager software is capable of expanding the size of a volume, but can only do so manually. An administrator must manually enlarge a volume's capacity by first adding additional physical disk drives and then using the software to configure each volume with the new storage. Since this method is manual, the administrator must add on enough disk space so that he does not need to perform this procedure frequently. Therefore this expansion technique also has the inherent disadvantage of having large amounts of unused disk space over a large period of time.